Bug fix: Render and 3d view conflict over ob->imat

* For rendering every object's inverse matrix "ob->imat" needs to
  be in render view coordinates, but for drawing in 3d view it needs
  to be in global coordinates. Originally (way back in historical
  times) ob->imat was only used for rendering, but over the years
  other uses came up too. Before 2.5 this wasn't a problem as
  rendering was a totally blocking operation, but with the new
  "interactive ui while rendering" the problems started. Basically
  any update that redrew the 3d view while rendering (like rotating
  the scene with mouse) updated the inverse matrix into global
  coordinates thus invalidating it for the render, leading to all
  kinds of strange discontinuities with textures and volumetrics.

* Problems were very easy to achieve using orco/object coordinates
  for pretty much any textures (bump, point density, volume, sky),
  for examples see bug reports 24906 and 25229. Render baking normals
  in object coordinates was wrong most of the time too.

* Now there is a separate inverse matrix ob->imat_ren that's
  calculated at the beginning of rendering and is used in all places
  inside render code where it's needed. This way the original ob->imat
  can change at will during rendering without causing problems.

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